Epoxy based adhesive compositions are reactive adhesive compositions comprising an epoxy resin, a curing agent and usually an accelerator. Upon heat-activation, the epoxy groups of the epoxy resin react with the curing agent linking the epoxy resin compounds by a polyaddition reaction to obtain a cured product. Such a cured product is known to have good mechanical properties and a chemical resistance superior to the cured product of other reactive adhesives. These characteristics make epoxy adhesive compositions particularly useful for demanding applications where stringent mechanical requirements must be satisfied, for example, in the automotive industry.
The cured product of an epoxy adhesive other than a structural epoxy adhesive generally has a relatively high static strength, for example, a high tensile and lap shear strength. Its dynamic strength however, i.e., its impact peel strength, is generally low. Adhesives used for the assembly of parts of a vehicle, such as cars, vans, lorries, trucks and trains, are called structural adhesives. The cured product of such a structural adhesive has to bear high static and dynamic loads. For that purpose, additional flexibilizers and/or tougheners are to be added to the epoxy adhesive composition.
Structural epoxy adhesives have been described in several patent applications: EP-A-0 197 892; (U.S. Pat. No. 4,659,779; U.S. Pat. No. 4,713,432; U.S. Pat. No. 4,734,332; U.S. Pat. No. 4,701,378, all incorporated herein by reference) describes a structural adhesive comprising an epoxy resin, a nitrogen comprising toughener and an accelerator. The problem described by this application is to provide a structural adhesive having a good storage stability and a high curing rate. EP-A-0 308 664 and (U.S. Pat. No. 5,278,257 all incorporated herein by reference) describes an epoxy adhesive composition comprising a butadiene-acrylonitrile copolymer in combination with a polyphenol-terminated polyurethane or polyurea. EP-A-0 353 190; (U.S. Pat. No. 5,073,601; U.S. Pat. No. 5,202,390, all incorporated herein by reference) describes an epoxy adhesive composition comprising a butadiene-acrylonitrile copolymer in combination with a polyether-, polyester, polythioester or polyamide prepolymer terminated with a functionalized carbocyclic aromatic or araliphatic residue. The technology according to EP-A-0 308 664 and EP-A-0 353 190 is called Mühlhaupt technology. The problem described by both EP-A-0 308 664 and EP-A-0 353 190 is to improve the impact peel strength of the cured product. At low temperatures however, the impact peel strength is low. WO 00/20483 and (U.S. Ser. No. 2004/81013 all incorporated herein by reference) relates to a composition comprising an epoxide-reactive copolymer having a glass transition temperature of −30° C. or less, and a reaction product of a carboxylic acid anhydride with a di- or polyamine and a polyphenol or aminophenol. The cured product of WO 00/20483 has an impact peel strength of less than 20 N/mm at −40° C.
Since vehicles are normally exposed to varying climatic conditions, the cured product of a structural adhesive is required to have good mechanical properties at room temperature as well as at low temperatures. Because of their low impact peel strength values at low temperatures, the above known epoxy adhesives do not fully meet this requirement. Vehicles, parts of which are assembled by the above known epoxy adhesives, consequently do not fully comply with safety requirements, for example, in a crash test. A further disadvantage of the above known epoxy adhesive compositions are their relatively low impact behaviour on high strength metals both at moderate and at extreme temperatures. Due to this disadvantage, the use of the known epoxy adhesive compositions for assembling the parts of a high strength metal construction is limited.
There is a need to provide an epoxy adhesive composition which upon curing results in a product having superior mechanical properties, in particular a high impact peel strength, at temperatures as low as about −40° C., while maintaining the desirable mechanical properties of conventional epoxy adhesive compositions at room temperature. There is a further need to provide an epoxy adhesive composition which upon curing results in a product having a high impact peel strength on high strength metals.